The invention relates to the field of electrical protection in general, and more particularly to the protection of thyristor-controlled reactors.
In electrical apparatus and on power lines, there is a need for protection against overvoltage, overcurrent, faults, surges and breakdowns. Protective measures range from remedial action to equipment shutdown. These may be triggered immediately, like a fuse or a switch, or they may involve initial steps of detection of the occurrence of the event with an appraisal being made of the size of the danger before taking any drastic action, like a shutdown.
The present invention lies in a small scale simulating circuit providing a live reproduction of the operative apparatus to be protected. It is particularly applicable to the thyristor-controlled reactors as are used for VAR compensation on power lines.
Protection of thyristor-controlled reactors against overcurrent is known from U.S. Pat. No. 3,989,999 of M. B. Brennen and F. T. Thompson. There, a predetermined delay intervenes before causing shutdown.
Another form of protection for thyristor-controlled reactor is the detection of insulation "leakage" or "flashover" current.
The thyristor-controlled reactor, as shown in the afore-mentioned Brennen and Thompson patent, involves reactors connected across the power lines through switches controlled for regulating the buildup of reactive power which effects power factor compensation in conjunction with capacitors on the high voltage transmission lines. Typically, the power switches are high voltage thyristor valves inserted between two high power reactors. This is known as a split reactor arrangement. The thyristor-controlled reactor (TCR) is usually connected to the secondary windings of the main transformer which belongs to the static VAR generator (SVG). Two reactors are used in order that the thyristor can react to limit the fault current as it develops through one reactor which fails, for instance by flashover, thereby avoiding on a single reactor a total shorted turns or flashover situation involving the overall TCR, so that very high fault currents would result that the valve would not be able to handle. The invention will protect the valve from being damaged under such circumstances. It is a protection which may be added to the other forms of protection in a static VAR generator, such as the afore-mentioned overcurrent and differential current protections for thyristor-controlled reactors.
In many applications, a static VAR generator (SVG) is connected to high voltage utility company transmission lines (typically, at more than 100 kv) to provide reactive power compensation. It is possible that the transmission line voltage increases significantly under certain conditions. This may cause an increase up to two per unit lasting for several line voltage cycles. Since in a static VAR generator, the thyristor-controlled reactor (TCR) is connected between lines, the TCR is exposed to such overvoltage condition. Therefore, a reactor failure may occur at any thyristor firing angle and voltage level under voltage conditions ranging from less than nominal to the one corresponding to a line voltage transient as specified for a given UGT installation.
Protection of the TCR must be provided for the worst overvoltage condition to be expected. First, failure of a reactor should be detected as soon as possible. If it is the case for one of the two reactors involved, trip should be initiated immediately to prevent the voltage stress from building up on the other reactor, thereby to protect the thyristor valve.
The problem in detecting reactor failure lies in that in the phase-controlled operation of the TCR, the current may remain below the nominal current level even when the line voltage increases above its nominal value. Therefore, with a fixed set point overcurrent detector, such as known in the prior art, reactor failure cannot be detected under all line voltage and phase angle conditions. For instance, with a current differential protection, that is, a system which senses the two currents on the respective reactors and detects a critical difference therebetween, there may be a difference of current detected if there is a leakage, for instance due to breakdown of an insulator on one side. Nevertheless, if there is a fully, or partially shorted reactor, this will not produce a differential current because the same current flows through the system. What is needed, is a detection system which is sensitive to both the voltage levels and the firing angles.
A solution to the problem, according to the present invention, consists in establishing a small scale electronic model of the TCR. The model is mounted across control level voltages that are proportional to the high voltage bus lines. Because it is an electronic model, it does not fail due to overvoltage like would the reactors in a true device, and it will always carry a current representing the acceptable operational current for a given firing angle of the thyristor valve and under the operative voltage. The small scale model operates as an equivalent circuit, like an observer, and provides a scaled down representative current which is matched in magnitude with the transformer sensed current of the operative TCR. Should a critical event, like a flashover, affect one of the reactors, the sensed current will no longer have the normal operation magnitude. Therefore, critical discrepancy will appear between the reference signal derived from the scaled model and the actual signal derived from the TCR.